The possibility of refining the grain structureof a commercial Al-Mg-Si alloy wasevaluated using asymmetric rolling (ASR) andaccumulative roll bonding (ARB) in the severeplastic deformation (SPD) regime. Bars ofannealed alloy having a thickness of 10 mmwere asymmetrically rolled down to athickness of 0.23 mm with a laboratoryrolling mill featuring the possibility ofindependently modifying the rotational speedof its two rolls. The effect of the rollingtemperature was investigated by tests in therange 150-250°C. A parallel campaign wasalso conducted to investigate the effects ofwarm accumulative roll bonding of the samealloy and in the same temperature range.These tests were carried out on annealedsamples of 1 mm thickness. The experimentalcharacterization (both mechanical andm i c ro s t ructural) demonstrated thatasymmetric rolling and accumulative rollbonding can readily promote the achievementof ultrafine grained structures in Al-Mg-Sialloys

Angella, G.

Dellasega, D.

Farè, S.

Vedani, M.

Italian Group Fracture (IGF): E-Journals / Gruppo Italiano Frattura

22 Metallurgical Science and Technology Vol. 28-1  -  Ed. 2010A comparison between asymmetric rollingand accumulative roll bonding as means torefine the grain structure of an Al-Mg-Si alloyG. Angella1, D. Dellasega2, S. Farè3, M. Vedani31National Research Council, Institute for Energetic and Interphases, Italy2Politecnico di Milano, Dipartimento di Energia, Italy3Politecnico di Milano, Dipartimento di Meccanica, ItalyABSTRACTThe possibility of refining the grain structureof a commercial Al-Mg-Si alloy wasevaluated using asymmetric rolling (ASR) andaccumulative roll bonding (ARB) in the severeplastic deformation (SPD) regime. Bars ofannealed alloy having a thickness of 10 mmw e re asymmetrically rolled down to athickness of 0.23 mm with a laboratoryrolling mill featuring the possibility ofindependently modifying the rotational speedof its two rolls. The effect of the rollingtemperature was investigated by tests in therange 150-250°C. A parallel campaign wasalso conducted to investigate the effects ofwarm accumulative roll bonding of the samealloy and in the same temperature range.These tests were carried out on annealedsamples of 1 mm thickness. The experimentalcharacterization (both mechanical andm i c ro s t ructural) demonstrated thatasymmetric rolling and accumulative ro l lbonding can readily promote the achievementof ultrafine grained structures in Al-Mg-Sialloys.RIASSUNTOLa possibilità di affinare la microstruttura diuna lega commerciale della serie Al-Mg-Si èstata valutata usando  le tecniche sia dilaminazione asimmetrica (ASR) che disaldatura per "accumulative roll bonding"(ARB) laminazioni ripetute (ARB), ambeduee ffettuate in regime di deform a z i o n eplastica severa (SPD). Le barre ricotte,spesse 10 mm, sono state laminate fino allos p e s s o re di 0,23 mm mediante unlaminatoio di laboratorio che consentiva  dimodificare in modo indipendente la velocitàdi rotazione dei due cilindri. L’effetto dellatemperatura di laminazione è stato inoltrestudiato con prove effettuate nell’intervallodi temperature tra 150-250°C. Nello stessointervallo di temperatura, parallelamente èstata condotta una campagna di prove pervalutare gli effetti di una laminazione ARB atiepido sulla stessa lega. Queste prove sonostate eseguite su campioni ricotti spessi1mm.  La caratterizzazione sia meccanicache micro s t rutturale ha mostrato cheambedue i sistemi di laminazione sono ingrado di produrre  nelle leghe Al-Mg-Sistrutture a grano ultrafine.KEYWORDSAluminum alloys, severe plastic deformation,ultrafine grained materials.Contribution presented to the 1st TMS-ABMInternational Materials Congress, Symposium“Mechanical Pro p e rties of Materials withEmphasis on Grain-size Effects”, July 26-30,2010, Rio de Janeiro, Brazil.23Metallurgical Science and TechnologyVol. 28-1  -  Ed. 2010INTRODUCTIONIt has been widely proved that a reductionin grain size corresponds to enhancedmechanical properties of metallic alloys[2]. The generation of Ultrafine Grained(UFG) materials, with a grain size in thesubmicrometer scale thus leads to materialswith higher strength and improved ductilityat room temperature compared to coarsegrained mate rials, as well as bet te rfo rmability at high te mp e ra t u re (underappropriate strain rate and temperaturesuperplastic behavior can easily occur) [3].One of the available mechanisms th a tallows strong refining of the grain structureis continuous dynamic re c rysta l l i z a t i o n(cDRX) [4]. According to this process, them a te rial ex p e riences the fo rmation ofdeformation/microshear bands for low andmedium equivalent strain, while at higherd e gree of st rain, in the Seve re Plast i cD e fo rmation (SPD) regime, gra i nsubdivision occurs due to the increase inb o u n d a ry misori e n tation, eve n t u a l lyleading to high-angle grain boundaries.Second phase particles are known [1] toease and accelerate the fragmentation ofshear bands, so allowing to reach an UFGstructure sooner, expecially for small andmedium sized particles.A l te rn a t i ve ly, many other th e rm o -m e chanical processes ex i st giving th epossibility to refine the grain structure bydiscontinuous recrystallization.However, the attempts to reduce grain sizein commercial aluminum alloys exploitingthis phenomenon have encountered manydifficulties due to its high stacking faultse n e rgy [5]. This ch a ra c te ri stic st ro n glyfa c i l i ta tes the phenomenon of re c ove ryinstead of recrystallization. Several SPDtechniques have been developed in lastdecades in order to impart high amounts ofequivalent strain to materials, either at labor at industrial scale, by exploiting cDRXm e chanism. In the fo rmer group, it ispossible to cite Equal Channel AngularE x t rusion (ECAE) and High Pre s s u reTorsion (HPT). The latter group includesb oth Asymmet ric Rolling (ASR) andAccumulative Roll Bonding (ARB). Rollingprocesses are among the most attractivep rocesses for SPD since th ey can bei n d u st ri a l ly ex p l o i ted with modestmodifications of the rolling mill. The aim ofthis wo rk is to comp a re two ro l l i n gtechniques (ASR and ARB) as a means torefine the grain structure of a 6082 alloy.In ASR, different peripheral speeds of theupper and lower rolls are adopted and thesheet is thus subjected to enhanced sheardeformation, in addition to compressionstrain. It has been suggested that this wouldre a d i ly pro m ote the development ofu l t ra fine grains. Cui and Ohori [6]investigated the structure evolution of high-purity aluminium during room temperatureASR with reduction of thickness exceeding90% and asymmet ry ratio of 1.4 (th emismatch speed ratio between upper andl ower ro l l s ) . T h ey sugge sted that th eevolution of microstructure results from thed evelopment of high-angle gra i nb o u n d a ries (HAGBs) due to th esimultaneous action of compression andshear st rain. Jin and Lloyd [7,8] alsoperformed ASR followed by annealing on acommercial 5754 aluminum alloy. Afterappropriate processing with an asymmetryratio of 2, they were able to produce grainsizes as small as 1 µm featuring a tensileresponse comparable to that of ultrafine-grained alloys produced by other SPDmethods.ARB is a discontinuous process that keepsthe ge o m et ry and size of the billet sconstant. It has been shown [9,10] that thefo rmation of UF grains during inte n s ep l a stic defo rmation is the simulta n e o u saction of grain subdivision and recovery of the deformed structure. Ani mp o rtant concern when exploiting th etechnique is that is possible to achieve are a s o n a b ly homogeneous micro st ru c t u reand tex t u re using a small number of passes.MATERIALS ANDEXPERIMENTAL PROCEDURESCommercial bars of a type 6082  alloy,having a thickness of 10 mm and width of40 mm, were annealed at 450°C for 30minutes and cold rolled down to a thicknessof 0.23 mm. This overall reduction wasachieved by a multipass procedure with noi n te rm e d i a te annealing treatments. Alaboratory rolling mill (roll diameter 150mm, width 200 mm) fe a t u ring th epossibility of independently modify th erotational speed of its two rolls wa sa d o pted for this purpose. The ro l l i n gschedule imposed a reduction of thicknessof 20% at each step and the rotation of theb i l l et along its longitudinal axis befo reeach pass. The asymmetry ratio adoptedwas 1.4 on the basis of previous studies[11]. To improve the formability of the alloyand to inve st i ga te the effects of wa rmproperties. Microstructure analyses werec a rried out by scanning electro nm i c ro s c o py (SEM) and tra n s m i s s i o nelectron microscopy (TEM).SEM and TEM samples were prepared bymechanically grinding the materials downto about 85 µm thickness. Final twin jetthinning was performed at 18V with asolution of 30 vol% HNO3 in methanol at -30°C. Results have been analyzed as afunction of equivalent strain, considering ast rain of 0.8 for each ARB pass( c o rresponding to 50% reduction inthickness) [12] and calculated according tothe equation (1) for ASR [13]:rolling, the process was conducted at250°C, with an inter-pass heating of 5m i n u tes. For accumulative roll bonding,b a rs of the same mate rial we rec o nve n t i o n a l ly rolled down to 1 mmthickness and then annealed with the sameheat treatment of the original samp l e s .Special surface pre p a ration wa sperformed to improve sheet bonding: wireb rushing and acetone washing wa sperformed before stacking the two sheetstogether by rivets. As for the ASR, the ARBwas performed at 250°C.The investigation here presented is mainlyfocused on micro h a rdness andm i c ro st ru c t u re as a function of ro l l i n gconditions. Micro h a rdness measure m e n t swith a load on the indenter of 1 N werep e rfo rmed to eva l u a te mech a n i c a lε = 1.155 ln (1)(  )h0hf24 Metallurgical Science and Technology Vol. 28-1  -  Ed. 2010RESULTS AND DISCUSSIONIn Figure 1 the evolution of microhardnessas a function of equivalent strain is plottedfor both processes.The two curves appear qu i te diffe re n t .While there is an initial stage of hardnessincrease for low and medium equivalentst rains, for ARB a decrease inmicrohardness occurs for high levels ofst rain. The behaviour of asymmet ri c a l lyrolled specimens is qu i te diffe re n t: th einitial increase is more modera te andreaches a maximum for equivalent strain ofabout 200%, then remains almost constant.The micro st ru c t u ral analysis allowe dunderstanding this behaviour consideringthe different structures that are formed inthe first part of the process. In Figure 2re p re s e n ta t i ve micro graphs are show n ,referring to samples with similar equivalentstrain.In the micro graphs it is clear th a ta s y m m et ri c a l ly rolled sample has th etendency to create microshear bands, whileARB gives rise to a more homogeneousmicrostructure. Furthermore, the dislocationdensity seems to be higher in Figure 2 a)and this evidence can be a goodexplanation for the higher value ofm i c ro h a rdness of the ARBed mate ri a l .Fu rther plastic st raining leads to aprogressive evolution of the microstructure.While asymmetrically rolled sample tendsto decrease the width of the shear bandsand fragment them, the ARBed material stillremains homogeneous and the apparentdislocation density increases. Thisbehaviour is well recognized in figure 3 a)and b).The analysis of Figure 3 a) and b) isi mp o rtant to understand the diffe re n tm e chanical behaviour of the samp l e sprocessed by the two methods at very highst rains. By considering the continuousi n c rease of dislocation density for th eARBed material, it is clearly explained whyit possesses higher value of microhardness.Instead, the asymmetrically rolled material,reduces the size of microshear bandswithout any increase in dislocation densityand this can be the reason why the curve inFigure 1 has a smoother evolution.Figure 4 refers to the last step of bothrolling process. The value of equivalentstrain is approximately the same (400% forARB and 438% for ASR) and th emicrostructures are quite similar as well.It is inte re sting to note that th eFig. 1: Evolution of microhardness as a function of equivalent strain for both ARB and ASR process at 250°C.Fig. 2: TEM Micrograph of ARBed a) and ASRed b) samples subjected to equivalent strain of 80% and77% respectively.A BFig. 3: TEM micrographs of the rolled samples; a) ARBed sample at 160% of equivalent strain; b) ASRedsample at 154% of equivalent strain.A Bm i c ro st ru c t u re of ASRed mate rial isdefinitely homogeneous, since the processof fragmentation already started at lowerstrains. Not only fragmentation, but alsorecovery occurred. In particular, recoveryeases the fragmentation of shear bands25Metallurgical Science and TechnologyVol. 28-1  -  Ed. 2010Fig. 4: TEM Micrographs of a) ARBed sample at 400% of equivalent strain and b) ASRed sample at438%of equivalent strain.A Band this implies the reduction of dislocationdensity. It is possible to state that both thereduction of grain size by fragmentationand of dislocation density by re c ove ryconcur to keep the microhardness constantfor high levels of equivalent strain.ARBed mate rial (Fig. 4 a)) shows, asexpected, a more homogeneous structure.The main difference with respect to theASRed material is the dislocation density.During the process the material underwenth e avy re c ove ry th et lowe rs mech a n i c a lp ro p e rties due to decrease in wo rkh a rdening. This evolution of the twomicrostructures reflects the microhardnesscurves for both materials.A n other imp o rtant fe a t u re that can bedrawn for both rolling processes is thestimulation of precipitation and coarseningof second phase particles due to holding at250°C and possibly to enhanced diffusionre l a ted to the heavy defo rm a t i o nexperienced. As depicted in Figures 5 and6, due to st rain and te mp e ra t u re, th efraction and size of second-phase particlesFig. 5: SEM BSE micrographs of ARBed samples with equivalent strain a) 80%, b) 160%, c) 400%.A B Ci n c rease dra m a t i c a l ly, preventing clearo b s e rvations of the grains with th eb a cks c a t te red electron ch a n n e l l i n gtechnique. Furthermore, coarsening of theparticles clearly occur by increasing thenumber of passes (i.e. the strain).Finally, Figure 6 confirms the formation ofmicroshear bands for the asymmetricallyrolled material at the beginning of therolling process, detected also by a differenttechnique. For the last sample (Fig. 6 c)) ahomogeneous microstructure is highlighted.Fig. 6: SEM BSE micrographs of ASRed asymmetrically rolled samples with equivalent strain of a) 77%, b) 154% and c) 438%.A B C26 Metallurgical Science and Technology Vol. 28-1  -  Ed. 2010CONCLUSIONSThe evolution of the microstructure stronglydepends on the process adopted. Fo rasymmetric rolling of the 6082 alloy, thefirst part of the process is spent to createmicroshear bands (up to an equivalentstrain of about 150%) and to reduce theirw i d th. For higher values of equ i va l e n tst rain, the mate rial underg o e sfragmentation of the microshear bands andthe subsequent creation of an ultra fi n estructure, even at a warm temperature of250°C. At the end of the process, recoveryand work hardening are almost balancedso that the mechanical properties do notchange significantly with further strain.ARB, due to a different state of strain, doesnot impose a severe orientation of thegrains and the microstructure generated ateach pass remains fairly homogeneous.The large st part of the evolution isc o n c e n t ra ted at the beginning of th ep rocess and the contribution of wo rkhardening is considerable, determining agreat response in te rms of mech a n i c a lproperties, as shown by the microhardnesscurve. At the end of the process, recoveryis more important than work hardening sothat the mate rial shows a decrease inmechanical properties when deformed at250°C to very large strains.REFERENCES[1] Evangelista, E, McQueen, HJ, Bonetti, E. Proceedings of the Riso International Symposiumon Metallurgy and Materials Science 1983:243.[2] H. Gleiter: Prog. Mater. Sci., 33 (1989), 223.[3] H. Pirgazi, A. Akbarzadeh, R. Petrov and L. Kestens: Mater. Sci. Engin., A 497 (2008), 132.[4] T. Sakai, H. Miura, A. Goloborodko and O. Sitdikov: Acta Mater., 57 (2009), 153.[5] Y. Huang and F.J. Humphreys: Acta Mater., 45 (1997), 4491.[6] Q. Cui and K. Ohori: Mat. Sci. Tech., 16 (2000),  1095.[7] H. Jin and D.J. Lloyd: Metal. Mater. Trans., 35A (2004), 997.[8] H. Jin and D.J. Lloyd: Scripta Mater., 50 (2004), 1319.[9] N. Kamikawa, T. Sakai and N. Tsuji: Acta Mater., 55 (2007), 5873.[10] S.G. Chowdhury, V.C Srivastava, B. Ravikumar and S. Soren: Scripta Mater., 54 (2006),1691.[11] S. Farè, M. Vedani and G. Angella: Mater. Sci. Forum, 604-605 (2009), 77.[12] Y. Saito, H. Utsunomiya, N. Tsuji and T. Sakai: Acta Mat., 47 (1999), 579.[13] L. Esteban, M.R. Elizalde and L. Ocana: J. Mater. Proc. Techn. 183 (2007), 390.

A comparison between asymmetric rolling and accumulative roll bonding as means to refine the grain structure of an Al-Mg-Si alloy

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A comparison between asymmetric rolling and accumulative roll bonding as means to refine the grain structure of an Al-Mg-Si alloy

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